Jen,
The gears in the car (1st and 2nd you mention) are what they are. That combined with the final drive ratio gives your gearing (mph/1000 revs). That doesn't change one bit with a pedal box. All the pedal box is doing is translating the movement of your foot on the accelerator differently to stock. It doesn't change the gearing at all.
On a diesel, due to the fact that they run very high compression ratios (amongst other reasons), most of the time they red line at between 4.5-5.5k revs. A petrol can go as high as 8k revs as it runs lower compression rates (and many other factors related to diesel vs petrol fuels). It's one of the reasons why my last 3 diesel VW's (one Scirocco and 2 MK7 GTD's) all had DSG boxes rather than manual. Because you run out of revs so quickly in 1st and 2nd gear, and when trying to make a quick getaway, rather than snatching for the next gear so quickly as the rev counter enters the red zone, the DSG just takes care of it, and allows you to make rapid progress without really noticing the limited rev range on the diesel engines.